1. Field of the Invention:
This invention relates to a phase-locked loop device, that is, a device for generating a clock synchronized in phase with the input clock.
2. Description of the Related Art:
The phase-locked loop (PLL) circuit has found its use in many purposes. For example, in the apparatus for recording and reproducing digital signals such as a digital video tape recorder (D-VTR), for the purpose of extracting data from the reproduced signal, it is indispensable to form a clock synchronized in phase with the reproduced signal.
In this specification, taking an example of such a PLL circuit as adapted to be used in the D-VTR, description is made. FIG. 1 is a diagram illustrating the construction of the conventional, general PLL circuit used in the D-VTR.
In the figure, the clock obtained from the reproduced signal and entering at an input terminal 301 is compared in phase by a phase comparator (PC) 302 with the clock Cl that is the output of a voltage-controlled oscillator (VCO) 304, and a phase comparison voltage S1 is produced. From the phase comparison voltage S1, the high-frequency component is removed by a loop filter 303. At the output of the loop filter 303, an error voltage S2 is produced. It is by the error voltage S2 that the frequency of the aforesaid clock Cl that is the output of the voltage-controlled oscillator (VCO) 303 is controlled.
By such a construction and arrangement of its parts as described above, the PLL circuit becomes able to produce the clock Cl synchronized in phase with the clock component of the reproduced signal at its output terminal 305. Based on this clock, data is extracted from the reproduced signal.
By the way, in a case where such a PLL circuit as described above is used as means for forming the clock for data extraction of the D-VTR, the lock range must be so widened that phase locking operates over a wide frequency range. This is because the frequency of the clock component in the reproduced signal changes when reproduction is performed while simultaneously transporting the tape at a different speed from that for the normal recording or reproduction mode, for example, when in the so-called special reproduction mode (cue, review, etc.) or the long-time reproduction mode.
The widening of the lock range, however, contains a problem that the stability of the PLL circuit is deteriorated. It has, therefore, been difficult to secure a wide frequency range for locking, while preserving the stability against the jitter required to the D-VTR, etc.